Latex foam is normally produced according to the state of the art by expanding a heat-vulcanizable composition, e.g., as generally defined below, with air or another gas, combining the composition with a gelling agent, such as sodium silicofluoride for example, and then vulcanizing the mixture by heat (German Patent 1,056,364; DOS 1,470,810). Gelling takes place at room temperature, in an infrared treatment zone, or in the heating-up phase for vulcanization. In the non-gel method, special measures for gelling are omitted. The thus-produced latex foam has a cellular structure.
Latex foam must exhibit a spectrum of good properties which can be described by the following combination of characteristics:
(a) high elasticity in a temperature range from 20.degree. to 70.degree. C., i.e. in this temperature range, after a relatively long compression period, the compression set is to be as low as possible (DIN 53,572); PA1 (b) high tensile strength and high elongation at break (DIN 53,571); and PA1 (c) the foam density, with a given indentation hardness, is to be as low as possible (DIN 53,576, economy); in other words with a predetermined foam density, the indentation hardness (compression resistance) is to be at a maximum. PA1 a. they are obtained by semicontinuous emulsion polymerization in the presence of 1-6, especially 3-5% by weight of a conventional fat soap and/or resin soap as the emulsifier and .ltoreq.0.3, especially .ltoreq.0.2% by weight of a conventional compound regulating the molecular weight of the polymer, based in each case on the entire amount of all monomers used to prepare the latex; PA1 b. the graft copolymer (A) comprises 35-65% (preferably 45-60%) by weight of a graft substrate (A1) (which latter, in turn, can comprise a graft substrate (B1) and grafted-on proportion (B2)), and 35-65% (preferably 40-55%) by weight of a grafted-on proportion (A2) (i.e. grafted-on in the last stage overall); PA1 c.1 the graft substrate (A1) is obtained in one stage; it comprises 86-95% (preferably 87-93%) by weight of repeating units obtained by polymerization of styrene in the presence of 1,3-butadiene (styrene units), and 5-14% (preferably 7-13%) by weight of repeating units obtained by polymerization of 1,3-butadiene in the presence of styrene (1,3-butadiene units); PA1 c.2 the graft substrate (A1) is obtained, as an alternative to c.1, in two stages as graft substrate B1 with grafted-on proportion B2; in the first stage, a polystyrene nucleating latex is obtained, as graft substrate B1; its solids are 1-20, especially 5-15% by weight, based on the final amount of graft copolymer A present at the end of the last stage; the grafted-on proportion (B2) grafted onto graft substrate (B1) in the second stage comprises 86-95 % (preferably 87-93%) by weight of repeating units obtained by polymerization of styrene in the presence of 1,3-butadiene (styrene units), and 5-14% (preferably 7-13%) by weight of repeating units obtained by polymerization of 1,3-butadiene in the presence of styrene (1,3-butadiene units); intensive agitation is performed while feeding the emulsifier during the grafting process in order to avoid formation of new particles; PA1 d. the proportion of the graft copolymer grafted on (in the last stage, A2) comprises 62-70% (preferably 65-69%) by weight of repeating units obtained by polymerization of styrene in the presence of 1,3-butadiene (styrene units), and 30-38% (preferably 31-35%) by weight of repeating units obtained by polymerization of 1,3-butadiene in the presence of styrene (1,3-butadiene units); intensive agitation is carried out while feeding the emulsifier during the grafting process in order to avoid formation of new particles; PA1 e. the graft copolymer (A) is present in the disperse phase of the latex; the latter, on account of features b through d, has comparatively hard and soft zones with a comparatively high and low glass transition temperature (Tg), respectively, in the individual particles, and has an average particle diameter of 120-300 (preferably 150-250)nm, determined by surface titration (soap titration). PA1 a reinforced elastomer component, PA1 an effective amount of a vulcanizing agent, and PA1 conventional additives; PA1 1 it is obtained by the following measures: PA1 1.1 an elastomer latex is mixed with a reinforcing latex (a latex of a reinforcing polymer; solids: 2-30 (preferably 5-25) parts by weight per hundred parts total (phr)); PA1 1.2 the mixture of latices is concentrated to a solids content of .gtoreq.55% (preferably .gtoreq.60%) by weight; PA1 2 the elastomer latex is an NR latex or an SBR latex which latter has been subjected to measures effecting agglomeration of the elastomer particles, or it is a mixture of an NR latex and an SBR latex which latter has been subjected to measures effecting agglomeration of the elastomer particles; PA1 3 the SBR latex is not carboxylated; it has been obtained by emulsion polymerization in the presence of a conventional fat soap and/or resin soap as emulsifier; the SBR contains 15-35% (preferably 20-30%) by weight of repeating units obtained by polymerization of styrene (styrene units); PA1 4 the reinforced elastomer component, at a solids content of 68% by weight, has a viscosity of .ltoreq.2,500 (preferably .ltoreq.2000) mPa.multidot.s, determined in a Brookfield viscosimeter (spindle III, 30 rpm, 20.degree. C.); PA1 j. the reinforcing latex is a styrene/butadiene graft copolymer latex of this invention as described above.
The above-described combination of properties is only inadequately realized by the state of the art. Such foams are especially useful in production of mattresses and upholstery.